The cryogenic rectification of air to produce oxygen and nitrogen is a well established industrial process. Typically the feed air is separated in a double column system wherein nitrogen shelf or top vapor from a higher pressure column is used to reboil oxygen bottom liquid in a lower pressure column.
The demand for lower purity oxygen is increasing in applications such as glassmaking, steelmaking and energy production. Less vapor boilup in the stripping sections of the lower pressure column, and less liquid reflux in the enriching sections of the lower pressure column are necessary for the production of lower purity oxygen which has an oxygen purity of less than 98.5 mole percent, than are typically generated by the operation of a double column.
Accordingly, lower purity oxygen is generally produced in large quantities by a cryogenic rectification system wherein feed air at the pressure of the higher pressure column is used to reboil the liquid bottoms of the lower pressure column and is then passed into the higher pressure column. The use of air instead of nitrogen to vaporize the lower pressure column bottoms reduces the air feed pressure requirements, and enables the generation of only the necessary boil-up in the stripping sections of the lower pressure column either by feeding the appropriate portion of the air to the lower pressure column reboiler or by partially condensing a larger portion of the total feed air.
While the conventional air boiling cryogenic rectification system would be effective for the production of lower purity oxygen, its ability to generate liquid nitrogen reflux for supply to the top of the lower pressure column is limited. This results from the lower component relative volatilities at the operating pressure of the higher pressure column which is similar to that of the main air feed and because of the large fraction of liquid air produced. More power is consumed because oxygen recovery is reduced as a result of the reduced capability to generate liquid nitrogen reflux.
Accordingly, it is an object of this invention to provide a cryogenic rectification system for producing lower purity oxygen wherein the liquid bottoms of a lower pressure column are reboiled by indirect heat exchange with feed air and which operates with reduced power requirements over that of conventional air boiling systems especially while producing oxygen at a concentration less than 90 mole percent.
Often it is desired to recover the product oxygen gas at an elevated pressure. Generally this is carried out by compressing the product gas to a higher pressure by passage through a compressor. Such a system is effective but is quite costly. Moreover, air boiling cryogenic rectification systems have heretofore been most useful for the production of lower pressure oxygen.
Accordingly, it is another object of this invention to provide an air boiling cryogenic rectification system which can effectively produce elevated pressure oxygen gas without the need for oxygen gas compression.